The present invention relates to a technique to receive electric power from a primary coil by means of a resonance circuit.
An example of a high-frequency electric power receiving circuit, which is disclosed in Japanese Unexamined Patent Application Publication No. H10-108390, will be explained with reference to FIG. 5. The high-frequency electric power receiving circuit includes a secondary coil 102, a resonance capacitor 103, a rectifier circuit 104, and a constant voltage control circuit 105. The secondary coil 102 faces a primary side induction line 101 (primary coil) that passes a high-frequency alternating current. The resonance capacitor 103 is connected in parallel to the secondary coil 102. The rectifier circuit 104 is connected to the resonance capacitor 103 and rectifies an induced voltage generated at the secondary side coil. The constant voltage control circuit 105 is connected to the rectifier circuit 104 and controls an output voltage VOUT, so that the output voltage VOUT is maintained at a predetermined and constant reference voltage VE.
The constant voltage control circuit 105 includes a coil 107, a voltage generator 108, a comparator 109, an output adjusting transistor 110, a diode 111, and a capacitor 112. The coil 107 is used for current limitation. The voltage generator 108 generates the reference voltage VE. The comparator 109 compares the output voltage VOUT with the reference voltage VE of the voltage generator 108. The comparator 109 turns on the output adjusting transistor 110 when the output voltage Vout exceeds the reference voltage VE. The diode 111 and the capacitor 112 form a filter. The reference voltage VE is set to the same voltage as a voltage (load voltage) to be generated between both ends of a load 106 when the load 106 is fed with electric power.
When the output voltage VOUT falls below the reference voltage VE, the output adjusting transistor 110 is turned off and, therefore, the output voltage VOUT is increased. When the output voltage VOUT exceeds the reference voltage VE, the transistor 110 is turned on by an output from the comparator 109 and, therefore, the output voltage VOUT is decreased. Due to such an operation of the constant voltage control circuit 105, the output voltage VOUT is maintained at the reference voltage VE.